Circuit Note CN-0411 Devices Connected/Referenced 8-Channel, Low Noise, Low Power, 24-Bit, AD7124-8 Sigma-Delta ADC with PGA and Reference Zero-Drift, Single-Supply, RRIO Op Amp AD8628 0.5 CMOS, 1.65 V to 3.6 V Dual SPDT/2:1 ADG836 Circuits from the Lab reference designs are engineered and MUX tested for quick and easy system integration to help solve todays JFET Input Instrumentation Amplifier with Rail- analog, mixed-signal, and RF design challenges. For more AD8220 to-Rail Output information and/or support, visit www.analog.com/CN0411. 0.5 CMOS Dual 2:1 MUX/SPDT ADG884 4.5 RON, 8-Channel 5 V, +12 V, +5 V, and ADG1608 +3.3 V Multiplexer Tiny 16-Bit SPI nanoDAC+, with 2 (16-Bit) LSB AD5683R INL and 2 ppm/C External Reference TDS Measurement System for Water Quality Monitoring The simplest method of measuring the conductivity of the solution EVALUATION AND DESIGN SUPPORT uses a 2-wire conductivity cell. Conductivity measurements Circuit Evaluation Boards require temperature compensation for measurements taken at CN-0411 Circuit Evaluation Board (EVAL-CN0411-ARDZ) temperatures other than 25C (or other reference temperature). ADICUP360 Development Board (EVAL-ADICUP360) This system can reference the conductivity measurement to Design and Integration Files room temperature using either a 100 or 1000 , 2-wire Schematics, Layout Files, Bill of Materials, Software resistance temperature device (RTD) and can accommodate CIRCUIT FUNCTION AND BENEFITS 2-wire conductivity cells of various cell constants and operating parameters. The total dissolved solids (TDS) present in a water system is composed of inorganic salts and small amounts of organic The capacitance and polarization effects of the electrodes in the matter that are dissolved in water, and is an important measure conductivity cell require that the excitation signal be a bipolar of water quality. TDS can be derived from the electrical square wave with a sufficiently high frequency to reduce conductivity (or conductivity) of the solution by a factor polarization effects but also with sufficiently long periods to dependent on the properties, temperature and, number of ions reduce capacitance effects. To avoid damaging the conductivity present. By measuring the conductivity of the solution, electrodes, the signal must have a very low to zero dc offset and determining the TDS of the system is faster, economical, and magnitude. less complicated in contrast to the more accurate gravimetric The circuit can measure the range of conductivity values from method. The latter method involves evaporating the water and 1 S to 0.1 S. A multiplexer switches between seven precision weighing the residue, which is applicable in laboratory settings resistors of different values to set the gain when measuring the but impractical in the field. conductivity probe signal. The system can automatically The circuit shown in Figure 1 is a TDS measurement system determine the gain setting of the conductivity measurement based on the conductivity of the solution. This design uses a through an auto-ranging procedure implemented in software. combination of components that allow for single-supply The system can also be calibrated in the high conductivity range operation, which minimize circuit complexity, making this to increase its accuracy suitable for low-power and portable instrument applications. Rev. 0 Circuits from the Lab reference designs from Analog Devices have been designed and built by Analog Devices engineers. Standard engineering practices have been employed in the design and construction of each circuit, and their function and performance have been tested and verified in a lab environment at room temperature. However, you are solely responsible for testing the circuit and One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. determining its suitability and applicability for your use and application. Accordingly, in no event shall Tel: 781.329.4700 www.analog.com Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) Fax: 781.461.3113 2018 Analog Devices, Inc. All rights reserved. CN-0411 Circuit Note Figure 1. Total Dissolved Solids Measurement System Simplified Schematic Rev. 0 Page 2 of 12 U3 AD7124-8 CS DOUT/RDY DIN SCLK DGND AIN0/IOUT R31 U1 R28 4. 02k AD5683R 3k AIN1 SDI VREF VREF C14 SCLK 1nF VOUT VDAC P3 C16 SYNC RTD 0.01F 3.3VD C15 ADICUP360 HEADERS 5V 1nF POWER, ANALOG, DIGI0, DIGI1 U4 TXB0108PWR AIN6 R29 GAIN 5V RESISTORS 3k R29 IOREF IOREF 3k VCCA U5 P0.4 or P0.5 P0.4 or P0.5 ADG1608 MISO1 MISO1 S8 S5 S4 S3 S2 S1 A2 EN S7 S6 P3 MOSI1 MOSI1 A1 P2 3.3VA VDD SCLK1 SCLK1 A0 P1 VSS/GND DD P0.5 or P1.0 P0.5 OR P1.0 PWM3 P1.4 P1.4 PWM2 3.3VA P1.3 P1.3 C9 R45 PWM1 1k 1000pF P1.2 P1.2 GND AIN7 U7 A2 ADG836 U5 5V AIN8 + AD8628 ADG884 IN1 IN2 C32 IN1 IN2 A1 R43 47F 3.3VD IOVDD AD8220 150 + S1A D1 3.3VA AVDD S1A R42 150 D1 S1B J1 GAIN = 10 VDAC AIN9 S1B CONDUCTIVITY REF VREF REFIN2(+) CELL S2A D2 S2A REFIN2() D2 R44 S2B 3.3VA S2B 150 AVSS C12 3.3VA VDD GND R46 1nF GND VDD 1k 3.3VA + A3 AD8628 C31 47F U2 3.3VD ADP7102ARDZ-3.3 5V VIN VOUT 3.3VA E1 EN SENSE ADJ 330 GND 16186-001